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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Using Square Brackets in Bash: Part 1)
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[#]: via: (https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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Using Square Brackets in Bash: Part 1
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======
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![square brackets][1]
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This tutorial tackle square brackets and how they are used in different contexts at the command line.
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[Creative Commons Zero][2]
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After taking a look at [how curly braces (`{}`) work on the command line][3], now it’s time to tackle brackets (`[]`) and see how they are used in different contexts.
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### Globbing
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The first and easiest use of square brackets is in _globbing_. You have probably used globbing before without knowing it. Think of all the times you have listed files of a certain type, say, you wanted to list JPEGs, but not PNGs:
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```
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ls *.jpg
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```
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Using wildcards to get all the results that fit a certain pattern is precisely what we call globbing.
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In the example above, the asterisk means " _zero or more characters_ ". There is another globbing wildcard, `?`, which means " _exactly one character_ ", so, while
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```
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ls d*k*
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```
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will list files called _darkly_ and _ducky_ (and _dark_ and _duck_ \-- remember `*` can also be zero characters),
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```
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ls d*k?
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```
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will not list _darkly_ (or _dark_ or _duck_ ), but it will list _ducky_.
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Square brackets are used in globbing for sets of characters. To see what this means, make directory in which to carry out tests, `cd` into it and create a bunch of files like this:
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```
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touch file0{0..9}{0..9}
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```
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(If you don't know why that works, [take a look at the last installment that explains curly braces `{}`][3]).
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This will create files _file000_ , _file001_ , _file002_ , etc., through _file097_ , _file098_ and _file099_.
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Then, to list the files in the 70s and 80s, you can do this:
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```
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ls file0[78]?
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```
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To list _file022_ , _file027_ , _file028_ , _file052_ , _file057_ , _file058_ , _file092_ , _file097_ , and _file98_ you can do this:
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```
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ls file0[259][278]
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```
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Of course, you can use globbing (and square brackets for sets) for more than just `ls`. You can use globbing with any other tool for listing, removing, moving, or copying files, although the last two may require a bit of lateral thinking.
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Let's say you want to create duplicates of files _file010_ through _file029_ and call the copies _archive010_ , _archive011_ , _archive012_ , etc..
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You can't do:
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```
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cp file0[12]? archive0[12]?
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```
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Because globbing is for matching against existing files and directories and the _archive..._ files don't exist yet.
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Doing this:
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```
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cp file0[12]? archive0[1..2][0..9]
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```
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won't work either, because `cp` doesn't let you copy many files to other many new files. Copying many files only works if you are copying them to a directory, so this:
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```
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mkdir archive
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cp file0[12]? archive
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```
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would work, but it would copy the files, using their same names, into a directory called _archive/_. This is not what you set out to do.
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However, if you look back at [the article on curly braces (`{}`)][3], you will remember how you can use `%` to lop off the end of a string contained in a variable.
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Of course, there is a way you can also lop of the beginning of string contained in a variable. Instead of `%`, you use `#`.
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For practice, you can try this:
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```
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myvar="Hello World"
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echo Goodbye Cruel ${myvar#Hello}
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```
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It prints " _Goodbye Cruel World_ " because `#Hello` gets rid of the _Hello_ part at the beginning of the string stored in `myvar`.
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You can use this feature alongside your globbing tools to make your _archive_ duplicates:
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```
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for i in file0[12]?;\
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do\
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cp $i archive${i#file};\
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done
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```
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The first line tells the Bash interpreter that you want to loop through all the files that contain the string _file0_ followed by the digits _1_ or _2_ , and then one other character, which can be anything. The second line `do` indicates that what follows is the instruction or list of instructions you want the interpreter to loop through.
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Line 3 is where the actually copying happens, and you use the contents of the loop variable _`i`_ **twice: First, straight out, as the first parameter of the `cp` command, and then you add _archive_ to its contents, while at the same time cutting of _file_. So, if _`i`_ contains, say, _file019_...
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```
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"archive" + "file019" - "file" = "archive019"
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```
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the `cp` line is expanded to this:
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```
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cp file019 archive019
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```
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Finally, notice how you can use the backslash `\` to split a chain of commands over several lines for clarity.
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In part two, we’ll look at more ways to use square brackets. Stay tuned.
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1
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作者：[Paul Brown][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.linux.com/users/bro66
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[b]: https://github.com/lujun9972
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[1]: https://www.linux.com/sites/lcom/files/styles/rendered_file/public/square-gabriele-diwald-475007-unsplash.jpg?itok=cKmysLfd (square brackets)
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[2]: https://www.linux.com/LICENSES/CATEGORY/CREATIVE-COMMONS-ZERO
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[3]: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash
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@ -0,0 +1,149 @@
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[#]: collector: (lujun9972)
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[#]: translator: (HankChow)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Using Square Brackets in Bash: Part 1)
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[#]: via: (https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1)
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[#]: author: (Paul Brown https://www.linux.com/users/bro66)
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在 Bash 中使用方括号 [第 1 节]
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======
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![square brackets][1]
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> 这篇文章将要介绍方括号及其在命令行中的用法。
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看完[花括号在命令行中的用法][3]之后，现在我们继续来看方括号（`[]`）在上下文中是如何发挥作用的。
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### <ruby>通配<rt>Globbing</rt></ruby>
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方括号最简单的用法就是通配。你可能在知道“通配”这个概念之前就已经通过通配来匹配内容了，列出具有相同特征的多个文件就是一个很常见的场景，例如列出所有 JPEG 文件：
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```
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ls *.jpg
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```
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使用<ruby>通配符<rt>wildcard</rt></ruby>来得到符合某个模式的所有内容，这个过程就叫通配。
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在上面的例子当中，星号（`*`）就代表“0 个或多个字符”。除此以外，还有代表“有且仅有一个字符”的问号（`?`）。因此
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```
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ls d*k*
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```
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可以列出 `darkly` 和 `ducky`，而且 `dark` 和 `duck` 也是可以被列出的，因为 `*` 可以匹配 0 个字符。而
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```
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ls d*k?
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```
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则只能列出 `ducky`，不会列出 `darkly`、`dark` 和 `duck`。
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方括号也可以用于通配。为了便于演示，可以创建一个用于测试的目录，并在这个目录下创建文件：
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```
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touch file0{0..9}{0..9}
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```
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（如果你还不清楚上面这个命令的原理，可以看一下[另一篇介绍花括号的文章][3]）
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执行上面这个命令之后，就会创建 `file000`、`file001`、……、`file099` 这 100 个文件。
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如果要列出这些文件当中第二位数字是 7 或 8 的文件，可以执行：
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```
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ls file0[78]?
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```
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如果要列出 `file022`、`file027`、`file028`、`file052`、`file057`、`file058`、`file092`、`file097`、`file098`，可以执行：
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```
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ls file0[259][278]
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```
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当然，不仅仅是 `ls`，很多其它的命令行工具都可以使用方括号来进行通配操作。但在删除文件、移动文件、复制文件的过程中使用通配，你需要有一点横向思维。
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例如将 `file010` 到 `file029` 这 30 个文件复制成 `archive010` 到 `archive029` 这 30 个副本，不可以这样执行：
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```
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cp file0[12]? archive0[12]?
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```
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因为通配只能针对已有的文件，而 archive 开头的文件并不存在，不能进行通配。
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而这条命令
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```
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cp file0[12]? archive0[1..2][0..9]
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```
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也同样不行，因为 `cp` 并不允许将多个文件复制到多个文件。在复制多个文件的情况下，只能将多个文件复制到一个指定的目录下：
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```
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mkdir archive
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cp file0[12]? archive
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```
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这条命令是可以正常运行的，但它只会把这 30 个文件以同样的名称复制到 `archive/` 目录下，而这并不是我们想要的效果。
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如果你阅读过我[关于花括号的文章][3]，你大概会记得可以使用 `%` 来截掉字符串的末尾部分，而使用 `#` 则可以截掉字符串的开头部分。
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例如：
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```
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myvar="Hello World"
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echo Goodbye Cruel ${myvar#Hello}
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```
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就会输出 `Goodbye Cruel World`，因为 `#Hello` 将 `myvar` 变量中开头的 `Hello` 去掉了。
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在通配的过程中，也可以使用这一个技巧。
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```
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for i in file0[12]?;\
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do\
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cp $i archive${i#file};\
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done
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```
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上面的第一行命令告诉 Bash 需要对所有 `file01` 开头或者 `file02` 开头，且后面只跟一个任意字符的文件进行操作，第二行的 `do` 和第四行的 `done` 代表需要对这些文件都执行这一块中的命令。
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第三行就是实际的复制操作了，这里使用了两次 `$i` 变量：第一次在 `cp` 命令中直接作为源文件的文件名使用，第二次则是截掉文件名开头的 `file` 部分，然后在开头补上一个 `archive`，也就是这样：
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```
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"archive" + "file019" - "file" = "archive019"
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```
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最终整个 `cp` 命令是这样的：
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```
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cp file019 archive019
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```
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最后，顺带说明一下反斜杠 `\` 的作用是将一条长命令拆分成多行，这样可以方便阅读。
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在下一节，我们会了解方括号的更多用法，敬请关注。
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--------------------------------------------------------------------------------
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via: https://www.linux.com/blog/2019/3/using-square-brackets-bash-part-1
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作者：[Paul Brown][a]
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选题：[lujun9972][b]
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译者：[HankChow](https://github.com/HankChow)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://www.linux.com/users/bro66
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[b]: https://github.com/lujun9972
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[1]: https://www.linux.com/sites/lcom/files/styles/rendered_file/public/square-gabriele-diwald-475007-unsplash.jpg?itok=cKmysLfd "square brackets"
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[2]: https://www.linux.com/LICENSES/CATEGORY/CREATIVE-COMMONS-ZERO
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[3]: https://www.linux.com/blog/learn/2019/2/all-about-curly-braces-bash
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